Global server load balancing

ABSTRACT

A global server load balancing (GSLB) switch serves as a proxy to an authoritative DNS communicates with numerous site switches which are coupled to host servers serving specific applications. The GSLB switch receives from site switches operational information regarding host servers within the site switches neighborhood. When a client program requests a resolution of a host name, the GSLB switch, acting as a proxy of an authoritative DNS, returns one or more ordered IP addresses for the host name. The IP addresses are ordered using metrics that include the information collected from the site switches. In one instance, the GSLB switch places the address that is deemed “best” at the top of the list.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No. 09/670,487 filed Sep. 26, 2000, now U.S. Pat. No. 7,454,500, issued Nov. 18, 2008, and titled Global Server Load Balancing, the entire contents of which are herein incorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to load balancing among servers. More particularly, the present invention relates to achieving load balancing by, in response to resolving a DNS query by a client, providing the address of a server that is expected to serve the client with a high performance in a given application.

2. Description of the Related Art

Under the TCP/IP protocol, when a client provides a symbolic name (“URL”) to request access to an application program or another type of resource, the host name portion of the URL needs to be resolved into an IP address of a server for that application program or resource. For example, the URL (e.g., http://www.foundrynet.com/index.htm) includes a host name portion www.foundrynet.com that needs to be resolved into an IP address. The host name portion is first provided by the client to a local name resolver, which then queries a local DNS server to obtain a corresponding IP address. If a corresponding IP address is not locally cached at the time of the query, or if the “time-to-live” (TTL) of a corresponding IP address cached locally has expired, the DNS server then acts as a resolver and dispatches a recursive query to another DNS server. This process is repeated until an authoritative DNS server for the domain (i.e. foundrynet.com, in this example) is reached. The authoritative DNS server returns one or more IP addresses, each corresponding to an address at which a server hosting the application (“host server”) under the host name can be reached. These IP addresses are propagated back via the local DNS server to the original resolver. The application at the client then uses one of the IP addresses to establish a TCP connection with the corresponding host server. Each DNS server caches the list of IP addresses received from the authoritative DNS for responding to future queries regarding the same host name, until the TTL of the IP addresses expires.

To provide some load sharing among the host servers, many authoritative DNS servers use a simple round-robin algorithm to rotate the IP addresses in a list of responsive IP addresses, so as to distribute equally the requests for access among the host servers.

The conventional method described above for resolving a host name to its IP addresses has several shortcomings. First, the authoritative DNS does not detect a server that is down. Consequently, the authoritative DNS server continues to return a disabled host server's IP address until an external agent updates the authoritative DNS server's resource records. Second, when providing its list of IP addresses, the authoritative DNS sever does not take into consideration the host servers' locations relative to the client. The geographical distance between the server and a client is a factor affecting the response time for the client's access to the host server. For example, traffic conditions being equal, a client from Japan could receive better response time from a host server in Japan than from a host server in New York. Further, the conventional DNS algorithm allows invalid IP addresses (e.g., that corresponding to a downed server) to persist in a local DNS server until the TTL for the invalid IP address expires.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an improved method and system for serving IP addresses to a client, based on a selected set of performance metrics. In accordance with this invention, a global server load-balancing (GSLB) switch is provided as a proxy for an authoritative DNS server, together with one or more site switches each associated with one or more host servers. Both the GSLB switch and the site switch can be implemented using the same type of switch hardware. Each site switch provides the GSLB switch with current site-specific information regarding the host servers associated with the site switch. Under the present invention, when an authoritative DNS server resolves a host name in a query and returns one or more IP addresses, the GSLB switch filters the IP addresses using the performance metrics compiled from the site-specific information collected from the site switches. The GSLB switch then returns a ranked or weighted list of IP addresses to the inquirer. In one embodiment, the IP address that is estimated to provide the best expected performance for the client is placed at the top of the list. Examples of suitable performance metrics include availability metrics (e.g., a server's or an application's health), load metrics (e.g., a site switch's session capacity or a corresponding preset threshold), and proximity metrics (e.g., a round-trip time between the site switch and a requesting DNS server, the geographic location of the host server, the topological distance between the host server and the client program). (A topological distance is the number of hops between the server and the client). Another proximity metrics is the site switch's “flashback” speed (i.e., how quickly a switch receives a health check result). The ordered list can also be governed by other policies, such as the least selected host server.

The present invention is better understood upon consideration of the detailed description of the preferred embodiments below, in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a global server load balancing configuration.

FIG. 2 illustrates in a flow chart one embodiment of the algorithm for selecting the “best” address from the list of addresses supplied by the authoritative DNS.

FIG. 3 is a block diagram showing the functional modules of GSLB switch 12 and site switch 18 a relevant to the global server load balancing function.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates one embodiment of the present invention that provides a global server load balancing configuration. As shown in FIG. 1, global server load balancing (GSLB) switch 12 is connected to Internet 14 and acts as a proxy to an authoritative Domain Name System (DNS) server 16 for the domain “foundrynet.com” (for example). That is, while the actual DNS service is provided by DNS server 16, the IP address known to the rest of the Internet for the authoritative DNS sever of the domain “foundrynet.com” is a virtual IP address configured on GSLB switch 12. Of course, DNS server 16 can also act simultaneously as an authoritative DNS for other domains. GSLB switch 12 communicates, via Internet 14, with site switches 18A and 18B at site 20, site switches 22A and 22B at site 24, and any other similarly configured site switches. Site switch 18A, 18B, 22A and 22B are shown, for example, connected to routers 19 and 21 respectively and to servers 26A, . . . , 26I, . . . 26N. Some or all of servers 26A, . . . , 26I, . . . , 26N may host application server programs (e.g., http and ftp) relevant to the present invention. These host servers are reached through site switches 18A, 18B, 22A and 22B using one or more virtual IP addresses configured at the site switches, which act as proxies to the host servers. A suitable switch for implementing either GSLB switch 12 or any of site switches 18A, 18B, 22A and 22B is the “ServerIron” product available from Foundry Networks, Inc.

FIG. 1 also shows client program 28 connected to Internet 14, and communicates with local DNS server 30. When a browser on client 28 requests a web page, for example, using a Universal Resource Locator (URL), such as http://www.foundrynet.com/index.htm, a query is sent to local DNS server 30 to resolve the symbolic host name www.foundrynet.com to an IP address of a host server. The client program receives from DNS server 30 a list of IP addresses corresponding to the resolved host name. This list of IP addresses is either retrieved from local DNS server 30's cache, if the TTL of the responsive IP addresses in the cache has not expired, or obtained from GSLB switch 12, as a result of a recursive query. Unlike the prior art, however, this list of IP addresses are ordered by GSLB switch 12 based on performance metrics described in further detail below. In the remainder of this detailed description, for the purpose of illustrating the present invention only, the list of IP addresses returned are assumed to be the virtual IP addresses configured on the proxy servers at switches 18A, 18B, 22A and 22B (sites 20 and 24). In one embodiment, GSLB switch 12 determines which site switch would provide the best expected performance (e.g., response time) for client 28 and returns the IP address list with a virtual IP address configured at that site switch placed at the top. (Within the scope of the present invention, other forms of ranking or weighting the IP addresses in the list can also be possible.) Client program 28 can receive the ordered list of IP addresses, and typically selects the first IP address on the list to access the corresponding host server.

FIG. 3 is a block diagram showing the functional modules of GSLB switch 12 and site switch 18 a relevant to the global server load balancing function. As shown in FIG. 3, GSLB 12 includes a GSLB switch controller 401, health check module 402, DNS proxy module 403, metric agent 404, routing metric collector 405, and site-specific metric collector 406. GSLB switch controller 401 provides general control functions for the operation of GSLB switch 12. Health check module 402 is responsible for querying, either periodically or on demand, host servers and relevant applications hosted on the host servers to determine the “health” (i.e., whether or not it is available) of each host server and each relevant application. Site-specific metric collector 406 communicates with metric agents in site-specific switches (e.g., FIG. 3 shows site-specific metric collector 406 communicating with site-specific metric agent 407) to collect site-specific metrics (e.g., number of available sessions on a specific host server). Similarly, routing metric collector 405 collects routing information from routers (e.g., topological distances between nodes on the Internet). FIG. 3 shows, for example, router 408 providing routing metric collector 405 with routing metrics (e.g., topological distance between the load balancing switch and the router), using the Border Gateway Protocol (BGP). DNS proxy module 403 (a) receives incoming DNS requests, (b) provides the host names to be resolved to DNS server 16, (c) receives from DNS server 16 a list of responsive IP addresses, (d) orders the IP addresses on the list received from DNS server 16 according to the present invention, using the metrics collected by routing-metric collector 405 and site specific collector 406, and values of any other relevant parameter, and (e) provides the ordered list of IP addresses to the requesting DNS server. Since GSLB switch 12 can also act as a site switch, GSLB switch 12 is provided site-specific metric agent 404 for collecting metrics for a site-specific metric collector.

In one embodiment, the metrics used in a GSLB switch includes (a) the health of each host server and selected applications, (b) each site switch's session capacity threshold, (c) the round trip time (RTT) between a site switch and a client in a previous access, (d) the geographical location of a host server, (e) the current available session capacity in each site switch, (f) the “flashback” speed between each site switch and the GSLB switch (i.e., how quickly each site switch responds to a health check from the GSLB switch), and (g) a policy called the “Least Response selection” (LRS) which prefers the site least selected previously. Many of these performance metrics can be provided default values. Each individual metric can be used in any order and each metric can be disabled. In one embodiment, the LRS metric is always enabled.

FIG. 2 illustrates in a flow diagram one embodiment of an optimization algorithm utilized by GSLB switch 12 to process the IP address list received from DNS server 16, in response to a query resulting from client program 28. As shown in FIG. 2, in act 100, upon receiving the IP address list from DNS server 16, GSLB switch 12 performs, for each IP address on the IP address list (e.g., host server 26I connected to site switch 18B), a layer 4 health check and a layer 7 check. Here, layers 4 and 7 refer respectively to the transport and application protocols in the Open System Interconnection (OSI) protocol layers. The layer 4 health check can be a Transmission Control Protocol (TCP) health check or a User Datagram Protocol (UDP) health check. Such a health check can be achieved, for example, by a “ping-like” operation defined under the relevant protocol. For example, under the TCP protocol, a TCP SYN packet can be sent, and the health of the target is established when a corresponding TCP ACK packet is received back from the target. In this embodiment, the layer 7 health check is provided for specified applications, such as the well-known HyperText Transport Protocol (HTTP) and the File Transfer Protocol (FTP) applications. If a host server or an associated application fails any of the health checks it is disqualified (act 102) from being the “best” site and may be excluded from the IP address list to be returned to client program 28. Since the health check indicates whether or not a host server or an associated application is available, the health check metric is suitable for use to eliminate an IP address from the candidates for the “best” IP address (i.e., the host server expected to provide the highest performance). After act 100, if the list of IP addresses consists of only one IP address (act 101), the list of IP addresses is returned to client program 28 at act 108.

After act 100, if the list of candidate IP addresses for the best site consists of multiple IP addresses, it is further assessed in act 102 based upon the capacity threshold of the site switch serving that IP address. Each site switch may have a different maximum number of TCP sessions it can serve. For example, the default number for the “ServerIron” product of Foundry Network is one million sessions, although it can be configured to a lower number. The virtual IP address configured at site switch 18B may be disqualified from being the “best” IP address if the number of sessions for switch 18B exceed a predetermined threshold percentage (e.g., 90%) of the maximum number of sessions. (Of course, the threshold value of 90% of the maximum capacity can be changed.) After act 102, if the list of IP addresses consists of only one IP address (act 103), the list of IP addresses is returned to client program 28 at list 108.

After act 102, if the IP address list consists of multiple IP addresses (act 103), the remaining IP addresses on the list can then be reordered in act 104 based upon a round-trip time (RTT) between the site switch for the IP address (e.g., site switch 18B) and the client (e.g., client 28). The RTT is computed for the interval between the time when a client machine requests a TCP connection to a proxy server configured on a site switch, sending the proxy server a TCP SYN packet, and the time a site switch receives from the client program a TCP ACK packet. (In response to the TCP SYN packet, a host server sends a TCP SYN ACK packet, to indicate acceptance of a TCP connection; the client machine returns a TCP ACK packet to complete the setting up of the TCP connection.) The GSLB Switch (e.g., GSLB switch 12) maintains a database of RTT, which it creates and updates from data received periodically from the site switches (e.g., site switches 18A, 18B, 22A and 22B). Each site collects and stores RTT data for each TCP connection established with a client machine. In one embodiment, the GSLB switch favors one host server over another only if the difference in their RTTs with a client machine is greater than a specified percentage, the default specified percentage value being 10%. To prevent bias, the GSLB) switch ignores, by default, RTT values for 5% of client queries from each responding network. After act 105, if the top entries on the list of IP addresses do not have equal RTTs, the list of IP addresses is returned to client program 28 at act 108.

If multiple sites have equal RTTs then the list is reordered in act 106 based upon the location (geography) of the host server. The geographic location of a server is determined according to whether the IP address is a real address or a virtual IP address (“VIP”). For a real IP address the geographical region for the host server can be determined from the IP address itself. Under IANA, regional registries RIPE (Europe), APNIC (Asia/Pacific Rim) and ARIN (the Americas and Africa) are each assigned different prefix blocks. In one embodiment, an IP address administered by one of these regional registries is assumed to correspond to a machine located inside the geographical area administered by the regional registry. For a VIP, the geographic region is determined from the management IP address of the corresponding site switch. Of course, a geographical region can be prescribed for any IP address to override the geographic region determined from the procedure above. The GSLB Switch prefers an IP address that is in the same geographical region as the client machine. At act 107, if the top two entries on the IP list are not equally ranked, the IP list is sent to the client program 28 at act 108.

After act 106, if multiple sites are of equal rank for the best site, the IP addresses can then be reordered based upon available session capacity (act 109). For example, if switch 18A has 1,000,000 sessions available and switch 22B has 800,000 sessions available, switch 18A is then be preferred, if a tolerance limit, representing the difference in sessions available expressed as a percentage of capacity in the larger switch, is exceeded. For example, if the tolerance limit is 10%, switch 18A will have to have at a minimum 100,000 more sessions available than switch 22B to be preferred. If an IP address is preferred (act 110), the IP address will be placed at the top of the IP address list, and is then returned to the requesting entity at act 108. Otherwise, if the session capacity does not resolve the best IP address, act 111 then attempts to a resolution based upon a “flashback” speed. The flashback speed is a time required for a site switch to respond to layers 4 and 7 health checks by the GSLB switch. The flashback speed is thus a measure of the load on the host server. Again, the preferred IP address will correspond to a flashback speed exceeding the next one by a preset tolerance limit.

In one embodiment, flashback speeds are measured for well-known applications (layer 7) and their corresponding TCP ports (layer 4). For other applications, flashback speeds are measured for user selected TCP ports. Layer 7 (application-level) flashback speeds are compared first, if applicable. If the application flashbacks fail to provide a best IP address, layer 4 flashback speeds are compared. If a host server is associated with multiple applications, the GSLB switch selects the slowest response time among the applications for the comparison. At act 112, if a best IP address is resolved, the IP address list is sent to client program 28 at act 108. Otherwise, at act 113, an IP address in the site that is least often selected to be the “best” site is chosen. The IP address list is then sent to client program 28 (act 108).

Upon receipt of the IP address list, the client's program uses the best IP address selected (i.e., the top of the list) to establish a TCP connection with a host server. Even then, if there is a sudden traffic surge that causes a host server to be overloaded, or if the host servers or the applications at the site become unavailable in the mean time, the site switch can redirect the TCP connection request to another IP address using, for example, an existing HTTP redirection procedure. The present invention does not prevent a site switch from performing load balancing among host servers within its sub-network by redirection using a similar mechanism.

To provide an RTT under the present invention described above, at the first time a client accesses an IP address, a site switch (e.g., site switch 22A of FIG. 2) monitors the RTT time—the time difference between receiving a TCP SYN and a TCP ACK for the TCP connection—and records it in an entry of the cache database. The RTT time measured this way corresponds to the natural traffic flow between the client machine and the host sever specified, rather than an artificial RTT based on “pinging” the client machine under a standard network protocol. Periodically, the site switches report the RTT database to a GSLB switch along with load conditions (e.g., number of sessions available). The GSLB switch aggregates the RTTs reported into a proximity table indexed by network neighborhood. (A network neighborhood is the portion of a network sharing a prefix of an IP address.) The GSLB switch can thus look up the RTT for a client machine to any specific host server, based on the client's network neighborhood specified in the client's IP address. From the accesses to the host servers from a large number of network neighborhoods, the GSLB switch can build a comprehensive proximity knowledge database that enables smarter site selection. In order to keep the proximity table useful and up-to-date, the GSLB switch manages the proximity table with cache management policies (e.g., purging infrequently used entries in favor of recently obtained RTTs). The proximity data can be used for all IP addresses served by each site switch.

While particular embodiments of the present invention have been shown and described it will be apparent to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspect and, therefore, the appended claims are to encompass within their scope all such changes and modifications. 

What is claimed is:
 1. A method performed by a load balancing switch, the method comprising: storing, by the load balancing switch, round trip time data for each of a plurality of site switches, wherein the round trip time data for a site switch in the plurality of site switches comprises a time for exchanging at least one message between the site switch and a client machine, each site switch in the plurality of site switches associated with one or more host servers; receiving, by the load balancing switch from a domain name server, a plurality of network addresses generated by the domain name server in response to a domain name query originating at the client machine, the plurality of network addresses comprising network addresses of multiple site switches from the plurality of site switches; ordering, by the load balancing switch, the plurality of network addresses based upon the round trip time data for the plurality of site switches to generate a first ordered list of the plurality of network addresses; and causing, by the load balancing switch, the first ordered list of network addresses to be communicated to the client machine.
 2. The method of claim 1 wherein, for a first site switch in the plurality of site switches, the round trip time data for the first site switch comprises time information for exchanging at least one message between the first site switch and the client machine.
 3. The method of claim 2 wherein the time information for exchanging at least one message between the first site switch and the client machine comprises information indicating a time interval between a time when the first site switch receives a connection request from the client machine and a time when the first site switch receives a connection acknowledgement from the client machine in response to an acknowledgment packet sent from the first site switch to the client machine in response to receiving the connection request.
 4. The method of claim 1 further comprising reordering, by the load balancing switch, the first ordered list based upon a set of performance metrics associated with the plurality of site switches other than the round trip time data to generate a second ordered list.
 5. The method of claim 4 wherein the set of performance metrics comprises geographical locations of the host servers associated with the plurality of site switches.
 6. The method of claim 1 wherein storing the round trip time data for each of a plurality of site switches comprises storing the round trip time data for the plurality of site switches in a table indexed by network neighborhood.
 7. The method of claim 1 wherein each site switch in the plurality of site switches acts as a proxy for the one or more host servers associated with the site switch, the one or more host servers associated with the site switch being reached through the site switch.
 8. The method of claim 1 further comprising periodically receiving, by the load balancing switch, the round trip time data for each of the plurality of site switches from the site switch, wherein the round trip time data received from a site switch comprises round trip time data collected by the site switch for each connection established between the site switch and the client machine.
 9. The method of claim 1 further comprising: prior to the domain name server receiving the domain name query, communicating the domain name query from the client machine to a local domain name server that is different from the client machine and from the domain name server generating the plurality of network addresses; and wherein the domain name query is caused by a program executed by the client machine.
 10. A system comprising: a load balancing switch comprising a memory, the load balancing switch configured to store round trip time data for each of a plurality of site switches, wherein the round trip time data for a site switch in the plurality of site switches comprises a time for exchanging at least one message between the site switch and a client machine, each site switch in the plurality of site switches associated with one or more host servers; wherein the load balancing switch is configured to receive a plurality of network addresses generated by a domain name server in response to a domain name query originated from the client machine, the plurality of network addresses comprising network addresses of multiple site switches from the plurality of site switches, order the plurality of network addresses based upon the round trip time data for the plurality of site switches to generate a first ordered list of the plurality of network addresses, and cause the first ordered list of network addresses to be communicated to the client machine.
 11. The system of claim 10 wherein, for a first site switch in the plurality of site switches, the round trip time data for the first site switch comprises time information for exchanging at least one message between the first site switch and the client machine.
 12. The system of claim 11 wherein the time information for exchanging at least one message between the first site switch and the client machine comprises information indicating a time interval between a time when the first site switch receives a connection request from the client machine and a time when the first site switch receives a connection acknowledgement from the client machine in response to an acknowledgment packet sent from the first site switch to the client machine in response to receiving the connection request.
 13. The system of claim 10 wherein the load balancing switch is configured to reorder the first ordered list based upon a set of performance metrics associated with the plurality of site switches other than the round trip time data to generate a second ordered list.
 14. The system of claim 13 wherein the set of performance metrics comprises geographical locations of the host servers associated with the plurality of site switches.
 15. The system of claim 10 wherein the load balancing switch is configured to store the round trip time data for the plurality of site switches in a table indexed by network neighborhood.
 16. The system of claim 10 wherein each site switch in the plurality of site switches acts as a proxy for the one or more host servers associated with the site switch, the one or more host servers associated with the site switch being reached through the site switch.
 17. The system of claim 10 wherein the load balancing switch is further configured to periodically receive the round trip time data for each of the plurality of site switches from the site switch, wherein the round trip time data received from a site switch comprises round trip time data collected by the site switch for each connection established between the site switch and the client machine.
 18. The system of claim 10 wherein: prior to the domain name server receiving the domain name query, the domain name query is communicated from the client machine to a local domain name server that is different from the client machine and from the domain name server that generates the plurality of network addresses; and the domain name query is caused by a program executed by the client machine.
 19. A switch comprising: a memory configured to store round trip time data for each of a plurality of network devices, wherein the round trip time data for a network device comprises a time associated with communication between the network device and a client machine; and wherein the switch is configured to: generate an ordered list of network addresses based upon the round trip time data for the plurality of network devices, the ordered list of network addresses comprising network addresses generated by a domain name server and received by the switch in response to a domain name query originated from the client machine; and communicate the ordered list of network addresses to the client machine.
 20. The switch of claim 19 wherein, for a first network device in the plurality of network devices, the time associated with communication between the first network device and the client machine specifies a time interval between a time when the first network device receives a connection request from the client machine and a time when the first network device receives a connection acknowledgement from the client machine in response to an acknowledgment packet sent from the first network device to the client machine in response to receiving the connection request.
 21. The switch of claim 19 wherein the switch is configured to reorder the first ordered list based upon a set of performance metrics associated with the plurality of network devices other than the round trip time data to generate a second ordered list.
 22. The switch of claim 21 wherein the set of performance metrics comprises geographical locations of one or more host servers associated with the plurality of network devices.
 23. The switch of claim 19 wherein the memory is configured to store a table comprising the round trip time data for the plurality of network devices, wherein the table is indexed by network neighborhood.
 24. The switch of claim 19 wherein each network device in the plurality of network devices acts as a proxy for one or more host servers associated with the network device, the one or more host servers associated with the site switch being reached through the site switch.
 25. The switch of claim 19 wherein the domain name query is caused by a program executed by the client machine.
 26. A method comprising: storing, by a network device, round trip time data for each of a plurality of site switches, each site switch associated with one or more host servers, wherein the round trip time data for a site switch comprises a time for exchanging at least one message between the site switch and a client machine; receiving, by the network device from a domain name server, a set of network addresses for a domain name query originating at the client machine and received by the domain name server, the set of network addresses comprising network addresses of multiple site switches from the plurality of site switches; generating, by the network device, an ordered list of network addresses by ordering the set of network addresses based upon the round trip time data stored for the plurality of site switches; and communicating, by the network device, the ordered list of network addresses to the client machine.
 27. The method of claim 26: wherein the domain name query is caused by a program executed by the client machine; and prior to the receiving, the domain name query is communicated from the client machine to a local domain name server that is different from the client machine and from the domain name server from which the set of network addresses is received by the network device.
 28. A method comprising: receiving, by a load balancing switch, from a domain name system (DNS) server, a plurality of network addresses generated by the DNS server in response to a domain name query originated at a client machine; ordering, by the load balancing switch, the plurality of network addresses based upon round trip time data for a plurality of site switches to generate an ordered list of network addresses, the round trip time data comprising, for each site switch in the plurality of site switches, a time for exchanging at least one message between the site switch and the client machine; and causing, by the load balancing switch, the ordered list of network addresses to be communicated to the client machine.
 29. The method of claim 28 wherein each site switch in the plurality of site switches is associated with one or more host servers that are reached through the site switch using one or more virtual IP addresses configured at the site switch.
 30. The method of claim 28 wherein the ordered list of network addresses comprises virtual IP addresses configured at the plurality of site switches.
 31. The method of claim 28: wherein the domain name query is caused by a program executed by the client machine; and prior to the receiving, the domain name query is communicated from the client machine to a local domain name server that is different from the client machine and from the domain name server from which the set of network addresses is received by the load balancing switch.
 32. A network device comprising: a memory configured to store a plurality of network addresses received by the network device from a domain name system (DNS) server, the plurality of network addresses generated by the DNS server in response to a domain name query originated at a client machine; and a set of one or modules configured to: order the plurality of network addresses based upon round trip time data for a plurality of site switches to generate an ordered list of network addresses, the round trip time data comprising, for each site switch in the plurality of site switches, a time for exchanging at least one message between the site switch and the client machine; and cause the ordered list of network addresses to be communicated to the client machine.
 33. The network device of claim 32 wherein each site switch in the plurality of site switches is associated with one or more host servers that are reached through the site switch using one or more virtual IP addresses configured at the site switch.
 34. The network device of claim 32 wherein the ordered list of network addresses comprises virtual IP addresses configured at the plurality of site switches.
 35. The network device of claim 32 wherein the domain name query is caused by a program executed by the client machine. 